Tubular gas heater



C. KELLER TUBULAR GAS HEATER Oct 3, 1939.

Filed June 28, 1938 I ll 4 III!!! I Patented Oct. 3,1939

UNITED STATES TUBULAR GAS HEATER Curt Keller, Zurich, Switzerland; assignor to Aktiengcsellschaft fiir Technische Studien, Zurich,

Switzerland Application June 28, 1938, Serial No. 216,238 In Switzerland July 8, 193? 8 Claims.

This invention relates to means for heating air and gases and more especially to gas heaters of the tubular type in which the heat energy of gases of combustion (flue gases) is utilized for the heating up of other gases or air. It is an object of this invention to provide means whereby the economy of such gas heaters is improved.

As is well known to persons skilled in the art, if industrial gases or air shall be heated to several hundred degrees centigrade, by means of gases resulting in the combustion of liquid or solid fuel, whilepassing through heat exchanger tubes arranged in a furnace near the furnace wall, the poor heat transfer inside the tubes frequently causes the temperature of the tube walls to rise unduly, and this. the more so, the higher the furnace temperature to which the tubes are exposed. If such gas heater or its burner are also supplied with highly preheated combustion air, the high temperature prevailing in the furnace is likely to injure the heater tubes.

It has already been proposed to return into the furnace part of the flue gases which have already given off part of their heat, and to mix them therein with fresh gases of combustion. It has however been found that if this is done, means adapted to the particular furnace chamber and to the burners must be provided if an eiiicient mixture shall be obtained. This is particularly necessary if air, preheated say to several hundred degrees centigrade, is fed to the burners. In order that in such a case the transmission of heat by direct radiation to the heat exchanger tubes extending alongside of the wall of the furnace chamber shall not be too great, the temperature of the flame must be reduced directly after forma-' tion of the flame.

The present invention relates to a tubular gas heater of the kind in which the heat exchanger tubes are traversed by the gases to be heated and highly preheated air is used forthe combustion of the fuel and a part of the flue gases is returned into the furnace chamber by positive action. The invention also includes a particularly efiicient mode of operting such heater, whereby the temperature in the 'fumace can be reduced to such an extent that the heat exchanger tubes are no longer heated to an unduly high temperature by the heat radiated from the heating gases passing through the furnace.

To this end the returning flue gases are divided into at least two currents before entering the furnace of the heater, one current flowing along those heat exchanger tubes which adjoin the furnace wall and are directly exposed to the heat radiated by the heating gases traversing the furnace, while the other current is introduced into the space directly 'above'the flame. The quantity of flue gases which is returned and mixed with the gases of combustion, is preferably so proportioned that the temperature of the heating gases about to enter the space between the heat exchanger tubes to be heated is already lowered to such an extent that the tubes along which these example.

' In the drawing- Fig. 1 is an axial section of the heater, while Fig. 2 is a cross-section on the line II-II in Fig. 1.

Fig. 3 is an axial section showingv a modified form of temperature control,

Fig. 4 being a cross-section on the line IV-IV in Fig. 3.

Referring to the drawing and first to Figs. 1 and 2, I is the central heating chamber and 2 and 3 are cylindrical concentric metal partitions extending coaxially from the bottom and top of the heating chamber, respectively, these partitions being arranged in staggered relation so as to cause the heating gases to pass over partition 2 and below partition 3, before rising in contact with the outer wall of the heater. The combustion chamber 6 proper into whichv project the burners 1, is surrounded by a cylindrical partition 5 formed with inclined intake ports 4. Preheated fresh air is supplied to the combustion chamber 6 in axial direction through a tube 8. In the heating chamber I a large number of heat exchanger tubes 9 are arranged alongside of the furnace wall. Only two such tubes are shown in Fig. 1, each of which is curved zigzag fashion so as to form two ascending branches connected by a descending branch, the three branches being arranged in a radial plane. All the tubes 9 are connected by their bottom ends to a common annular distributing tube III for the gas to be heated and at the top to a common annular heater pipe H for the heated gas.

i The heating gases pass through the combustion chamber 6 and the heating chamber I in the direction of the arrows A. Part of them escapes through the tube I2 while the remainder is drawn oil. by a blower l2, through a tube It containing a throttle l4 and is returned into the heater I through a tube It. The returned heating gases are conducted into the heater I in two currents, one of which passes through an annular chamber l6 surrounding the partition 5 and rises in the heater in contact with inner ascending branches of the heater tubes 9 which are directly exposed to the heat radiated from the heating gases rising in the chamber I, so that a substantially cylindrical curtain of cooler gas is formed in front of these tubes, which protects them from direct contact with the heating gases.

This gas current may be imparted a spiral move- 1 ment in a helical path by guide vanes I'I arranged in the annular space It.

The second current of returned heating gases passes into the cylindrical combustion chamber 6 through the ports 4 in the partition 5, which are so formed and arranged that thissecond p'art current enters the combustion chamber 6 sym-.

metrically to its axis and directly above the heating flames issuing from the burners I. The passages for the returned heating gases are so dimensioned that the quantity of gas which passes through the annular space Iii is about one fourth of the quantity passing through the ports 4 into the combustion chamber 6. An annular slide valve l8 surrounding the partition 5 and formed with slots l9 (Fig. 2) serves for regulating the 'quantity of returned heating gas, admitted to the combustion chamber 6. The valve I8 is operated by a thermostat device 20 extending into the chamber I and controlling the operation of a motor H, which is connected by a gearing 2l and a system of rods 22 with the valve l8.

In the gas heater above described, the heating gases entering the combustion chamber 6 form a cooler gas layer above the flames, through which layer the hot flame gases are compelled to pass, mixing with the cooler gases, so that the temperature of the gas-mixture thus formed at the entrance to the heating, chamber proper is lowered uniformly, for instance down to 1000-1200 C. In consequence thereof the inner branches .of the tubes 9, which are already protected by the cooler gases issuing from the annular space l6 and have a temperature of for instance 500 to 700 C., can no longer be heated to an unduly high temperature by the heat radiated from the heating gases rising in the chamber l.

The quantities of heating gases returned into the heater and the admissible heating temperatures depend largely from the properties and admissible stressing of the materials used in the construction of the heater tubes.

InFigs. 3 and 4 the thermostat 23 is shown to Z positioned in direct contact with the inner branches of gas tubes 9, being thus enabled to control and regulate the passage of returning heating gases through the ports 4 and annular conduit l0, respectively, in accordance with the temperature of the tube walls. 1

Various changes may be made in the details disclosed in the foregoing specification without departing from the invention or sacrificing the advantages thereof.

1 claim:

1. A gas heater comprising in combination, a heating chamber, gas heating tubes havin ascending branches, tubes arranged along the periphery of said heating chamber, a combustion chamber below said heating chamber, means for returning into the heater heating gases, which have already passed through said heating chamber, and means for subdividing said returned gases into two substantially concentric layers, the inner layer entering said chamber directly above the flames so as to envelop them before they issue from said combustionchamber, while the outer layer rises in contact with the inner ascending branches of said gas heating tubes.

2. The gas heater of claim '1, including a perforated wall surrounding the combustion chamber and acting as a subdividing means, said wall being formed with ports for the introduction of the part of the returned gases designed to form the inner layer, and means provided for varying the cross-sectional area of said ports.

3. The gas heater of claim 1, including a periorated wall enclosing the combustion chamber and surrounded by an annular conduit for the passage of that part of the returned gases which shall form the outer layer. a

4. The gas heater of claim 1, including guide vanes adapted to impart to the returned gases spiral motion arranged in the path of 'at least one of the currents of returned gases.

5. The gas heater, of claim 1, including an adjustable damper for controlling the quantity of returned gases passing along the heat exchanger tubes. V

6. The gas heater of claim 1 in combination with means for regulating the subdivision of the returned gases in response to the temperature in the heating chamber.

'7. The gas heater of claim 1 in combination with means for regulating the subdivision of the returned gases in response to the temperature of the heat exchanger tubes.

8. The gas heater of claim 1 in combination with a thermostat device extending into the heating chamber, a valve governing the passage of returned gases into the combustion chamber and a motor controlled by said thermostat device and operatively connected with said valve.

CURT KELLER. 

